HIV1 performs highly complex orchestrated tasks during the assembly, budding, maturation, and infection stages of the viral replication cycle. During viral assembly, the proteins form membrane associations and self-associations that ultimately result in budding of an immature virion from the infected cell. Gag precursors function during viral assembly to selectively bind and package two plus strands of genomic RNA. HIV1 p17 is the matrix protein of the Gag polyprotein. Capsid protein p24 probably forms the conical core of the virus that encapsulates the genomic RNA-nucleocapsid complex. gp41/120 is the major HIV protein associated with the HIV envelope. It functions as the viral antireceptor or attachment protein. gp41 (or TM) traverses the envelope, whereas gp120 is present on the outer surface and is noncovalently attached to gp41. Upon activation of the envelope glycoprotein (gp120/41) by cellular receptors, gp41 undergoes conformational changes that mediate fusion of the viral and cellular membranes.